This invention relates to biomagnetometers, and, more particularly, to the verification of the proper operation of a biomagnetometer.
The biomagnetometer is an instrument that has been developed for measuring magnetic fields produced by the body, particularly the brain. The biomagnetometer is necessarily a very sensitive measurement instrument, because the magnetic fields produced by the brain are small. The strength of the magnetic field produced by the brain is typically about 0.000000001 Gauss, at a distance of 1-2 centimeters from the head. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or about five hundred million times larger than the strength of the magnetic field of the brain, as measured externally to the head.
The biomagnetometer includes a magnetic pickup coil connected to a very sensitive detector of magnetic signals. The currently most widely used detector is a Superconducting QUantum Interference Device or SQUID, which, in combination with a superconducting pickup coil, is sufficiently sensitive to detect magnetic signals produced by the brain. (See, for example, U.S. Pat. Nos. 4,386,361 and 4,403,189, whose disclosures are incorporated by reference, for descriptions of two types of SQUIDs.) The detector, pickup coil, and their associated equipment require special operating conditions such as a cryogenic dewar, and cannot be placed into the body or attached directly to the surface of the body. The dewar is operated with its interior at liquid helium temperature (about 4.2 K), to maintain the SQUID detector, the pickup coil, and the electrical connection between them in the superconducting state because of the small electrical currents involved, and to reduce the electrical noise that might otherwise influence the SQUID detector.
Special electronics is provided to filter out external effects such as the earth's magnetic field and other electrical sources. (For a description of the electronics, see U.S. Pat. Nos. 3,980,076 and 4,079,730, whose disclosures are herein incorporated by reference.) The subject and detector can also be placed into a magnetically quiet enclosure that shields the subject and the detector from the external magnetic fields. (For a description of such an enclosure, see U.S. Pat. No. 3,557,777, whose disclosure is herein incorporated by reference.) With these special provisions, medical researchers and doctors can now make accurate, reliable measurements of the magnetic fields produced by the brain either spontaneously or responsive to external stimuli.
Not only is it important to measure the field strength produced by the brain, but it is important to know the location in the head from which the magnetic field arises. The position of the head relative to the dewar, and thence the pickup coil, can be measured by an electromagnetic positioning system constructed so as not to interfere with the operation of the detector. The automatic position indicating system is described in U.S. Pat. No. 4,793,355, whose disclosure is incorporated by reference. A current version of the electromagnetic position indicating system can record the location of a magnetic source to within about 2 millimeters.
Because the position measurement system and the magnetic field measurement system of the biomagnetometer are complex in construction and operation, it is important to have a methodology for verifying that both are operating correctly. Only when proper operation is verified can the results be given their most meaningful interpretation. There have been proposed several techniques for such verification, including a "phantom" approach wherein a model head is provided with an internal source. However, for various reasons these prior techniques have not proved themselves to be sufficiently precise to verify the operation of the improved biomagnetometer systems today available.
There is therefore a need for an improved approach for verifying the operation of a biomagnetometer. The present invention fulfills this need, and further provides related advantages.